As a light spot image position sensitive device, the PSDs are widely used. In the PSDs, photo currents generated at the light incident position are detected through the resistive layer, the detected current values are calculated in analog form to rapidly obtain a center position of the light spot.
FIG. 4a shows a cross section of PSD, and FIG. 4b shows its equivalent circuit. A resistive layer R, a photo conductive layer P and a bias layer C make up the PSD. When light L is incident, photo currents are generated in the photo conductive layer P at the incident position. These photo currents enter into the resistive layer R at the light incident position due to an electric field produced by a bias electrode T.sub.cb and flow through the resistive layer in opposite directions. From output electrodes T.sub.1 and T.sub.2 provided at opposite ends of the PSD, the photo currents are obtained as output currents I.sub.a and I.sub.b. In the equivalent circuit shown in FIG. 4B, the photo currents entered into the resistive layer R from the light incident position are divided according to the ratio of resistance value between the light incident position and the output terminal T.sub.1 to that between the output terminal T.sub.2 and the light incident position. From the divided currents outputted from the respective output terminals as output currents I.sub.a and I.sub.b, a deviation coefficient of resistance values between the light incident position and the center position of resistance can be calculated from the following formula. ##EQU1##
Assume that the resistivity of the resistive layer is uniform, the resistance value is proportional to the length. Since the above calculated value X indicates the deviation coefficient of the light incident position from the center of the resistive layer, the light incident position can be obtainable by multiplying the length of the resistive layer R with the deviation coefficient. This calculation is usually carried out by such an analog operational circuit that a selection switch S.sub.w. is eliminated from the circuit shown in FIG. 3D, that is, the output currents I.sub.a and I.sub.b pass through a buffer amplifier B, two operational amplifiers .SIGMA. and a divider D. The obtained analog value is converted to a digital value X.sub.d by an analog-digital converter A/D. The relative resolution in image position sensing, that is, relative resolution of the image position sensitive section greatly depends on the accuracy of the analog signal processing circuit and also it is restricted by the resolution of the analog signal processing circuit and the analog-digital converter. A resolution of about 1/1000 can be easily obtained, but obtaining higher resolution is expensive and it is difficult to attain a resolution higher than 1/10,000. RIKEN hybrid type semiconductor image position sensitive device (hereinbelow referred to as R-HPSD) has been provided in Japanese Patent Application Disclosure No. 64-10108. This device has a plurality of output electrodes and increases the relative resolution in image position sensing by a factor of ten while the resolutions of the analog signal processing circuit and the A/D converter are unchanged.
FIG. 3A to FIG. 3D shows a concept of the R-HPSD which increases the relative resolution in image position sensing more than ten times without increasing the accuracy of the analog signal processing system. FIG. 3A shows a cross section of the R-HPSD in which output electrodes T.sub.2 . . . T.sub.5 are further provided between the electrodes T.sub.1 and T.sub.2 of FIG. 4A. Since other features of the R-HPSD are the same as the conventional PSD of FIG. 4A, those features will not be described in detail herein. In FIG. 3B, the output electrodes (T.sub.1 and T.sub.6) at the opposite ends are selected to detect in which one of the sections light is incident. Next, the electrodes at the both ends of the light incident section are selected to detect an image position within the selected section. The image position over the whole sensitive region is determined by adding the detected position within the selected section to the position of the selected electrode. The positions of the output electrodes are precisely settled by use of the technology for fabricating integrated circuits. The image position within the selected section is detected with a resolution defined by the accuracy of the analog signal processing system. Therefore, the resolution over the whole sensitive region is increased by a factor of the number of the sections compared with the conventional PSD. FIG. 3C shows output signals indicating image position obtained from output currents I.sub.a and I.sub.b of the electrodes straddling the whole region (S.sub.16) between the electrodes T.sub.1 and T.sub.6 or an individual section (S.sub.12. . . . . S.sub.56) among the electrodes T.sub.1 to T.sub.6, that is, (I.sub.b -I.sub.a)/(I.sub.a +I.sub.b). FIG. 3D shows the concept of a output electrode selection circuit and an analog signal processing circuit. The output electrodes are selected by the selection switch circuit S.sub.w. Since subsequent processing to obtain the light incident position within a section is the same as in the case of the conventional PSD shown in FIGS. 4A and 4B, the processing will not be described again.
Now, in the R-HPSD, the larger of the number of the output electrodes, that is, the larger of the number of the sections, the more advantageous to increase the relative resolution. However, when the number of the output electrodes is increased, the number of the switch elements of the output electrode selection circuit is also increased. Thus, if the switch elements are provided at the outside of the image position sensitive device, the number of output terminals leading to the switch elements is extremely increased. The output electrode selection circuit may be integrated with the image position sensitive device and this is ideal, but to develop an integrated circuit of the analog switch circuits greatly increases the costs.
As described above, in the R-HPSD, the relative resolution in sensing is improved by increasing the number of the output electrodes. However, a problem is arises regarding the output electrode selection circuit. In other words, switch elements for selecting the output electrodes are needed and structure and the control of the output electrode selection circuit are complicated. As a result, a new disadvantage is created, thereby reducing the inherent simplicity of the PSD.